The present invention is directed to integrated circuits. More particularly, the invention provides systems and methods for driving cold-cathode fluorescent lamps (CCFLs). Merely by way of example, the invention has been applied to intelligent control of one or more CCFLs. But it would be recognized that the invention has a much broader range of applicability.
Cold-cathode fluorescent lamps (CCFLs) are widely used for backlighting of thin-film-transistor (TFT) liquid-crystal displays (LCDs), such as television displays, computer displays, portable DVD displays, global positioning system (GPS) displays, handheld video-game console displays, and industrial instrument displays. The CCFLs often each include a sealed glass tube that contains one or more inert gases, such as Neon (Ne) and Argon (Ar) gases, which are also mixed with certain amount of mercury (Hg) vapor. Additionally, the sealed glass tube usually is internally covered by one or more fluorescent materials. If a high-magnitude and high-frequency AC voltage is applied to a cold-cathode fluorescent lamp (CCFL), the mercury vapor can be excited by the electric field, thus causing the CCFL to emit light.
FIG. 1 is a simplified diagram showing a conventional control system for one or more CCFLs. The control system 100 includes a power train component 110, a current/voltage feedback component 120, a controller chip 130, and a dimming-control interface 134. The controller chip 130 receives a dimming-control signal 135 from the dimming-control interface 134, and in response generates one or more gate drive signals 112. The power train component 110 receives the gate drive signals 112, and in response converts a direct-current (DC) voltage 114 generated from a DC power supply 136 to an alternating-current (AC) voltage 116. For example, the power train component 110 uses a voltage boost transformer and a resonant LC network to generate the AC voltage 116. The AC voltage 116 that is applied to the one or more CCFLs 132 is converted to a voltage-sensing signal 124 (e.g., Vvs) by the current/voltage feedback component 120. The voltage-sensing signal 124 is received by the controller chip 130, which generates the gate drive signals 112 and regulates the AC voltage 116 to a predetermined magnitude and a predetermined frequency. For example, the AC voltage 116 corresponds to different predetermined magnitudes and/or different predetermined frequencies for an ignition operation and a normal operation of the control system 100. As an example, the controller chip 130 includes an error amplifier. In another example, an output signal, Vcmp, of the error amplifier is used to determine a duty cycle of the gate drive signals 112 and thus the power transmitted to the one or more CCFLs 132. In yet another example, if the output signal, Vcmp, becomes higher, the power transmitted to the one or more CCFLs 132 also becomes higher.
As shown in FIG. 1, a current that flows through the one or more CCFLs 132 is also converted to a current-sensing signal 122 (e.g., Vcs) by the current/voltage feedback component 120. For example, the current/voltage feedback component 120 includes a current sensing resistor. In another example, the current-sensing signal 122 (e.g., Vcs) is also received by the controller chip 130 and compared with a first threshold (e.g., Vth1). In yet another example, if the current-sensing signal 122 (e.g., Vcs) becomes larger than the first threshold (e.g., Vth1), the control system 100 switches from the ignition operation to the normal operation. If the current-sensing signal 122 (e.g., Vcs) has not yet become larger than the first threshold (e.g., Vth1) but the voltage-sensing signal 124 (e.g., Vvs) is larger than a second threshold (e.g., Vth2) and/or the output signal Vcmp is larger than a third threshold (e.g., Vth3), the control system 100 keeps checking the current that flows through the one or more CCFLs 132 for a first predetermined period of time (e.g., T1).
If, during the first predetermined period of time (e.g., T1), the current-sensing signal 122 (e.g., Vcs) becomes larger than the first threshold (e.g., Vth1), the control system 100 switches from the ignition operation to the normal operation. If, during the first predetermined period of time (e.g., T1), the current-sensing signal 122 (e.g., Vcs) remains smaller than the first threshold (e.g., Vth1), the control system 100 shuts down the output of the AC voltage 116.
For example, the current that flows through the one or more CCFLs 132 after successful ignition is determined as follows:
                              I          CCFL                =                              V            in                    ×          N          ×                      2            π                    ×                      sin            ⁡                          (                                                π                  2                                ⁢                D                            )                                ×                                                1                              R                -                                  4                  ⁢                                      π                    2                                    ⁢                                      RCLf                    2                                                  +                                  j                  ⁢                                                                          ⁢                  2                  ⁢                  π                  ⁢                                                                          ⁢                  fL                                                                                                    (                  Equation          ⁢                                          ⁢          1                )            
where ICCFL represents the current that flows through the one or more CCFLs 132 after successful ignition. Additionally, Vin represents the magnitude of the DC voltage 114, and f represents the frequency of the AC voltage 116. Moreover, C represents the parasitic capacitance of the one or more CCFLs 132. Also, N, D, R, and L are constant parameters that are determined by the control system 100.
As discussed above, the AC voltage 116 can change in magnitude and/or in frequency if the control system 100 switches from the ignition operation to the normal operation. For example, the ignition of the one or more CCFLs 132 often needs the AC voltage 116 to be about 1000 volts in magnitude, but the normal operation of the one or more CCFLs 132 usually needs a much smaller magnitude for the AC voltage 116. In another example, each of the one or more CCFLs 132 has a high resistance level of about 10 MΩ before ignition but a much lower resistance level of about 200 KΩ at normal operation after successful ignition.
Also, as discussed above, the power train component 110 uses the voltage boost transformer and the resonant LC network to generate the AC voltage 116. For the resonant LC network, the voltage gain as a function of the voltage frequency often changes if the one or more CCFLs are successfully ignited.
FIG. 2(A) is a simplified diagram showing a conventional voltage gain of the resonant LC network used by the power train component 110 before successful ignition of a CCFL. As shown in FIG. 2(A), a waveform 200 represents the voltage gain of the resonant LC network as a function of voltage frequency. For example, the voltage gain reaches a peak value 202 at a resonant frequency 208 as shown by the waveform 200. In another example, the resonant frequency 208 is about 60 kHz.
FIG. 2(B) is a simplified diagram showing a conventional voltage gain of the resonant LC network used by the power train component 110 after successful ignition of the CCFL. As shown in FIG. 2(B), a waveform 204 represents the voltage gain of the resonant LC network as a function of voltage frequency. For example, the voltage gain reaches a peak value 206 at a resonant frequency 210 as shown by the waveform 204. In another example, the resonant frequency 210 is less than 50 kHz.
As shown in FIGS. 2(A) and 2(B), the resonant frequency 208 before successful ignition of the CCFL is significantly higher than the resonant frequency 210 after successful ignition of the CCFL (e.g., because of different electrical characteristics of the CCFL before and after the ignition).
Returning to FIG. 1, in order for the resonant LC network to achieve a high gain for both the ignition operation and the normal operation, the control system 100 may change the voltage frequency when the control system switches from the ignition operation to the normal operation. For example, during the ignition operation, the predetermined frequency of the AC voltage 116 is set higher and then, during the normal operation, is set lower after the detection of successful ignition of the one or more CCFLs 132.
Additionally, after the control system 100 enters into the normal operation, the controller chip 130 may compare the current-sensing signal 122 (e.g., Vcs) with an open-loop threshold (e.g., Vth—olp). If the current-sensing signal 122 (e.g., Vcs) is determined to be smaller than the open-loop threshold (e.g., Vth—olp) for a predetermined open-loop period of time (e.g., Tolp), the control system 100 may trigger the open loop protection (OLP) and shuts down the output of the AC voltage 116.
But the control system 100 may not function properly under certain circumstances. Hence, it is highly desirable to improve the techniques of controlling CCFLs.